Import existing chart releases

[apenney]

As an engineering team we've deployed a bunch of one-off helm install's across our three clusters. We've got a "provisioning" doc with several 'helm install' commands (as many people probably do).

Some of these charts can't be easily torn down and replaced, such as ingress controllers, as they create external resources that DNS is pointing to.

We'd like an option to describe the state of the cluster and then "import" existing releases of charts so that landscaper can take over management of them without disruption of existing services.